Charging electrodes are critical components used in electrostatic precipitators (ESPs), which are devices used to collect particles from gas streams, such as the streams from electric power plants burning coal. Examples of such devices are shown in U.S. Pat. No. 6,231,643 to Pasic, et al., United States Patent Application Publication No. US2008/0190296 published Aug. 14, 2008, and United States Patent Application Publication No. US2012/0227588 published Sep. 13, 2012, all of which are incorporated herein by reference.
The most basic ESP contains a row of wires followed by a stack of spaced, planar metal plates. A high-voltage power supply transfers electrons from the plates to the wires, developing a negative charge of thousand of volts on the wires relative to the collection plates. In a typical ESP, the collection plates are grounded, but it is possible to reverse the polarity.
The gas stream and particles flow through the spaces between the wires, and then pass through the rows of plates. During this, the gases are ionized by the charging electrode, forming a corona. As particles are carried through the ionized gases, the particles become negatively charged. When the charged particles move past the grounded collection plates, the strong attraction causes the particles to be drawn toward the plates until there is impact. When the particles contact the grounded plate, they give up electrons, and thus act as part of the collector. Automatic “rapping” systems and hopper evacuation devices remove the collected particulate matter while the ESPs are being used, thereby allowing ESPs to stay in operation for long periods of time.
Discharge electrodes have been developed that include rigid structures to which many sharpened spikes are attached, maximizing corona production. ESPs perform better if the corona is stronger and covers most of the flow area so particles cannot flow around the charging zones and escape being charged, which is called “sneakage”.
Conventional discharge electrodes are supported on a metal structure, which typically includes a support rod. The rods are conductive in order to electrically connect each spike point with the power supply. Generally, it is considered necessary to have metal spikes that can withstand the electrical currents that often flow due to sparking over between the collection substrate and discharge electrode. In corrosive operating conditions, the sharp spikes of the charging electrodes are also typically made of an expensive alloy (e.g., HASTELLOY brand alloy) to avoid or mitigate corrosion in the harsh environments in which such electrodes are used. Since the entire discharge electrode, including the rod, is commonly made of the same alloy, the electrodes become expensive and heavy, thereby requiring strong support structures.
Polymers are inexpensive, light and corrosion-resistant, but they do not conduct electricity, and they have poor tensile/flexural strength. Even conductive composites have much lower conductivity than metals. Therefore, the need exists for a discharge electrode that is lightweight and inexpensive, but still has a sufficient current flow and particle collection efficiency.